There is always a need for more bandwidth in communications channels, to accommodate a larger number of users. The finite or limited availability of channel bandwidth, in turn, makes the efficient use of bandwidth an economic necessity. The transmission of speech signals over limited-bandwidth channels has been the subject of extensive investigation and improvement. These improvements have given rise to devices known in the art as vocoders. In general, vocoders include a transmitter which analyzes the voice signal to be transmitted, and extracts various characteristics of the speech. These characteristics are encoded in some fashion, and transmitted over the limited-bandwidth transmission channel to a vocoder receiver. The vocoder receiver receives the encoded signals, and reconstitutes the original voice signal.
The voice signals which are reconstituted by the vocoder receiver never include all of the information occurring in the original voice signal, because the bandwidth of the transmission channel is incapable of carrying all of the information in the original voice. Thus, the quality of the signal received at the output of a vocoder system depends in part upon the bandwidth of the channel over which the signal must be transmitted, and in part upon the efficiency with which the system analyzes and reconstitutes the voice.
Of necessity, there is a certain amount of distortion in transmission over a vocoder system, and this distortion is manifested as coding noise. Various schemes have been advanced for masking or reducing the perceived amplitude of the coding noise. Among these schemes are those described in U.S. patent applications filed on Jul. 13, 1998, Ser. No. 09/114,658 in the name of Grabb et al.; Ser. No. 09/114,660 in the name of Zinser et al.; Ser. No. 09/114,661 in the name of Zinser et al. Ser. No. 09/114,662 in the name of Grabb et al.; Ser. No. 09/114,663 in the name of Zinser et al.; Ser. No. 09/114,664, in the name of Zinser et al.; and Ser. No. 09/114,659 in the name of Grabb et al., in which the amplitudes of the fundamental and its harmonics in the synthesized signal are increased or decreased in amplitude in response to the pole frequencies of the linear predictive coding (LPC) filter. In this arrangement, the general shape of the frequency spectrum represented by the coded signals remains the same, but the amplitude spread between the maximum-amplitude and minimum-amplitude components is adjusted (either increased or decreased).
Improved vocoder arrangements are desired.